A number of techniques and arrangements have been devised for coin pay station telephones to signal coin deposits to either operators or coin deposit signal receivers. The labor saving advantage of the latter is rather obvious. However, automatic electronic recognition of coin deposit signals in the presence of speech and noise has left something to be desired in terms of reliability compared to the trained human operator. For example, speech and/or noise may produce sufficiently strong frequency components which duplicate the tone burst portion of a coin deposit signal and are indistinguishable therefrom. This phenomenon is generally known as a simulation or as a "talk-off". A converse effect called "talk-down" prevents the detection of the tone burst portion of the coin deposit signal by the presence of sufficient signal energy at other frequencies than that of the tone burst signal.
Automatic detection is further hampered by the nature of the most common type of coin deposit signal which includes mechanically generated timing intervals of one or more tone bursts each followed by an appropriate silent interval. All of these intervals are subject to the imprecise timing of mechanical devices. Each deposited coin is indicated by a specific sequence format. More specifically, a deposited nickel is signaled by one tone burst followed by a silent interval. A first stepping period, which includes a tone burst and subsequent silent interval, followed by a second tone burst, indicate a deposited dime. Finally, a quarter deposit produces four similar stepping periods of shorter duration followed by a fifth tone burst and subsequent silent interval.
The generally imprecise mechanical arrangements in the pay stations which generate these timing intervals have their operation affected by wide swings in humidity, ambient temperature variations, and mechanical wear experiences by repeated use. All of the foregoing problems must be accounted for to detect coin tone deposit signals reliably by electronic means.
Another factor to be considered in automated coin deposit signal detection is the possibility of deceptive practices utilized to defraud the telephone operating company of toll charges for telephone calls from pay station telephones. Fortunately, the manner in which telephone connections are established allows the telephone company to identify valid coin deposit signals on the basis of signal levels present in different portions of transmission paths. Due to the uncontrolled signal ambient in these telephone connections, a straightforward signal level comparison technique is not always reliable.
It is an object of my invention to detect signal sequences automatically by providing a tolerance to interruptions appearing in both the tone burst and silent intervals of the sequence.
A further object is to utilize a level comparison of signal sequences with a margin of tolerance to signal inteference in different portions of transmission paths in a telephone system to identify the location at which the signal sequence originated to ascertain the validity thereof.